Sounds to me like it's not in general permitted (but with a huge exemption), but is possibly permitted to the extent that failure to comply with the GDPR constitutes unfair competition. So that means you can't simply use the "Abmahnen" procedure to enforce privacy rights, but rather need to demonstrate you're a market competitor and that it's relevant to your competitive position. Edit: no, I think I misread- that may be a possible conclusion but it's just not clear.
Sounds to me like it's not in general permitted (but with a huge exemption), but is possibly permitted to the extent that failure to comply with the GDPR constitutes unfair competition. So that means you can't simply use the "Abmahnen" procedure to enforce privacy rights, but rather need to demonstrate you're a market competitor and that it's relevant to your competitive position. Edit: no, I think I misread- that may be a possible conclusion but it's just not clear.
IANAL and all that.